This invention relates to throwaway inserts for rotary cutting tools for machining automotive parts and machine parts, particularly throwaway inserts suitable for drills, and throwaway drills carrying such inserts and improved in cutting balance, hole cutting accuracy and chip disposability.
A throwaway drill disclosed in examined Japanese patent publication 61-21766 carries throwaway inserts and is economical because their plurality of corners can be used for cutting, and is high in cutting balance. Referring to FIG. 12, this drill carries two hexagonal throwaway inserts 20 with alternating acute and obtuse corners. The inserts are arranged diametrically opposite to each other with one of the inserts placed nearer to the rotation axis of the drill than is the other. An angular cutting edges 25 of the insert nearer to the rotation axis is used as an inner cutting edge for cutting the diametrically inner half portion of a workpiece, while an angular cutting edge 25 of the insert remote from the rotation axis is used as an outer cutting edge for cutting the diametrically outer half portion of the workpiece.
Another conventional drill uses, instead of the inserts of FIG. 12, inserts 21 as shown in FIG. 13 as its inner and outer cutting edges in the same arrangement for performing the same function as the inserts of FIG. 12.
Another conventional drill carries throwaway inserts 22 as shown in FIG. 14 as inner and outer cutting edges. Each insert 22 has a protrusion 26 at the center of each cutting edge adapted to cut into the workpiece before does the cutting edge.
The applicant of the present invention also proposed another type of drill in examined Japanese patent publication 60-1928.
Still another type of conventional drill utilizes throwaway inserts 23 as shown in FIG. 15 as its inner and outer cutting edges. The insert 23 has substantially a rectangular contour and are formed with convex cutting edges 27 at both ends of the short sides.
The insert shown in FIG. 12 has cutting edges 25 each comprising two slopes intersecting each other at the apex. The two slopes are of the same inclination angle, so that the horizontal component of the cutting resistance that acts on each of the two slopes is cancelled out by the horizontal component that acts on the other slope.
The drill of examined Japanese utility model publication 60-1928 has its inner and outer inserts inclined in opposite ways to each other to cancel out the horizontal components of the cutting resistances that act on the respective inserts. These drills have a problem in that during high-efficiency (high-speed, high-feed) cutting or when a hard workpiece is machined, the cutting resistance tends to be unstable, which leads to ill-balanced cutting, which in turn makes it extremely difficult to form holes with accuracy or increases the possibility of breakage of the inserts and the drill. In particular, if a large load is abruptly applied when the inserts bite a workpiece during high-feed cutting requiring high drill rigidity or while a small-diameter, deep hole is being formed, the cutting balance tends to deteriorate markedly, resulting in breakage of the drill.
Cutting speed is zero at the center of rotation of a drill. The farther from the rotation axis, the greater the cutting speed. This means that the cutting resistance varies with the distance of the cutting edge from the rotation axis. The cutting resistance also varies with the cutting conditions and the kind of workpiece. If a conventional drill having an inner insert used to cut the diametrically inner part of the workpiece and an outer insert used to cut its diametrically outer part, is used for high-efficiency or high-load cutting, it is difficult to maintain good cutting balance due to too large a difference between the cutting forces produced by the inner and outer inserts.
The cutting area is divided into as many sections as the number of inserts used. Thus, even if a plurality of inserts are used, rather wide chips tend to be produced. Therefore, if a difficult-to-break material such as soft steel is machined at high speed or high feed rate, chips are produced in large amounts, increasing the possibility of the drill being clogged with chips. This is another cause of decreased machining accuracy and breakage of the drill.
When a hole is formed in soft steel, large burrs tend to be produced at the hole inlet if the diametrically outer cutting edge angle is large.
With the drill equipped with throwaway inserts as shown in FIG. 14, the protrusion 26 bites a workpiece before does the base portion of the cutting edge to bear the horizontal component of the cutting resistance. But this arrangement still has the problem of a large difference between the cutting forces produced by the inner and outer inserts. This drill also has many other problems including complicated shape of the seats for supporting the inserts 22, low strength of the protrusions 26, and high possibility of breakage of the insert and the drill especially during high-efficiency or high-load machining.
Further, since chips flow out simultaneously from three areas, i.e. the protrusion 26 and the straight portions of the cutting edge on both sides of the protrusion, chips tend to clog the drill by tangling with each other.
The drill having inserts as shown in FIG. 15 can apparently dispose of chips smoothly because one of the convex cutting edges 27 of one insert cuts a portion which the concave portion of the cutting edge of the other insert has passed. But this insert has thin portions between the concave portions (which are between the convex cutting edge portions) and the clamp hole. Thus, the insert is not strong enough as a whole. This means that the insert is likely to break during high-efficiency, high-load machining.
An object of this invention is to provide a high-performance throwaway drill which is free of any of the abovementioned problems.